1. Field of the Invention
The present invention relates to a method of printing an image by a printer having an asymmetric printer resolution, said method comprising the step of multi-level halftoning the image by assigning a level out of a number of levels to each pixel of the image. The present invention also relates to a printer, which is able to apply the method according to the present invention.
2. Background of the Invention
Printers are known, which are able to print an image on a receiving medium and comprise a processor unit for controlling print parameters with respect to printing the image and for performing calculations like halftoning with respect to printing of the image.
Such print parameters may, for example, be a velocity of a carriage on which the print head is positioned, a jet frequency with which ink drops are ejected from printing elements of the print head and a drop size of the ink drops ejected from printing elements of the print head, if a printer with the capability of ejecting ink drops of different drop sizes is used.
Nowadays, print heads of printers are using smaller and smaller sizes of ink drops and higher and higher jet frequencies. This is advantageous, since a sharper, less dingy image is achieved with the same productivity. To stay at a same level of productivity, the velocity of the carriage may not increase too much, since the productivity will decrease because of the large amount of time due to the reversal time of the carriage. To overcome this problem, an image may be printed by a higher resolution in the direction of a movement of the carriage than in the paper step direction. This results in an asymmetric printer resolution of for example 2400 dpi by 300 dpi, meaning a resolution of 2400 dpi in the direction of the movement of the carriage and a resolution of 300 dpi in the paper step direction. Known halftoning algorithms cannot easily manage such extreme asymmetric print resolutions. Artifacts may appear by which line structures become visible in the direction of the movement of the carriage.
A solution for this problem may be found in generating different pulse forms in the driving electronics of the print heads to create ink drops of different sizes. Such a pulse form is often a concatenation of driving pulses to achieve a large enough ink drop. The jet frequency will decrease and thus the velocity of the carriage may decrease. However, such a design of the driving electronics is cumbersome and control over the outcome of a concatenation of driving pulses is limited.